ISOTOPIC ESTIMATION OF CO2 PRODUCTION DURING EXERCISE BEFORE AND AFTER ENDURANCE TRAINING

Endurance training reduces the rate of CO2 release (i.e., Vco2) during submaximal exercise, which has been interpreted to indicate a reduction in carbohydrate oxidation. However, decreased ventilation, decreased buffering of lactate, and/or increased fixation of CO2 could also account for a lower Vco2 after training. We therefore used a primed continuous infusion of (NaHCO3)-C-13 to determine the whole body rate of appearance of CO2 (Ra(CO2)) in seven men during 2 h of cycle ergometer exercise at 60% of pretraining peak O2 uptake (VO2peak) before and after endurance training. Ra(CO2) is independent of the above-described factors affecting Vco2-but may overestimate net CO2 production due to pyruvate carboxylation and subsequent isotopic exchange in the tricarboxylic acid cycle. Training consisted of cycling at 75-100% Vo2peak for 45-90 min/day, 6 days/wk, for 12 wk and increased Vo2peak by 28% (P < 0.001). CO2 during submaximal exercise was reduced from 86.8 +/- 3.7 to 76.2 +/- 4.2 mmol/min, whereas Ra(CO2) fell from 88.9 +/- 4.0 to 76.4 +/- 4.4 mmol/min (both P < 0.001). Vco2 and Ra(CO2) were highly correlated in the untrained (r = 0.98, P < 0.001) and trained (r = 0.99, P < 0.001) states, as were individual changes in Vco2 and Ra(CO2) with training (r = 0.88, P < 0.01). These results support the hypothesis that endurance training decreases CO2 production during exercise. The magnitude and direction of this change cannot be explained by reported training-induced alterations in amino acid oxidation, indicating that it must be the result of a decrease in carbohydrate oxidation and an increase in fat oxidation. In addition, the greater decrease in Ra(CO2) than in Vco2 with training suggests that training may reduce the rate of pyruvate carboxylation during exercise.